Sometimes I forget to turn off time tracking on 1 or more items and then start tracking time on a new item. Things can also happen with members of my team. But we can only do one thing at a time.
I do realize that there are examples, in theory, where someone might be doing two related things at the same time and might therefore want to track time for 2 items at the same time.
That said, please at least provide an alert letting the user know, that there is a related item where they are currently tracking time and would they like to stop tracking time on the other item before tracking time on the new item?

I agree with above.
At teamwork.com they solve it by having a little indicator on the bottom of the screen that shows the current time tracking. The widget stays where ever you navigate in the app.
Once you start a new time tracking the previous pauses.
The recent tracked tasks stay in the widget wich makes it easy to switch between the recent task.
Often you switch between the same task during one day so i think something like this could be pretty handy…
UI and UX of teamwork.com is horrible (I try to switch our company to Monday) but this little feature is great!